A plurality of versions of a sewing unit called an integrated sewing station for working in extra width in a fabric layer have been known from the PFAFF information brochure "PFAFF 3811," printed in April 1989. During this sewing operation, which is also called pre-gathering, the fabric layer is crimped or gathered by the combined feeding of a main feeder operated with a basic stitch length and at least one differential feeder performing larger feed steps than the main feeder, and this state is fixed by a seam. A stabilizing ribbon is optionally sewn on as well.
The degree of pre-gathering or the degree of gathering can be set by a differential stitch regulator. Depending on the equipment of the machine, the differential stitch regulator is actuated either manually or by an adjusting element, which can be driven electrically and which is associated with twenty sensor buttons, which each represent a defined gathering value. In a second version, two preselected gathering values can be alternately selected during the sewing process by actuating a manual button. In a third version, programs with a larger number of different gathering values can be stored, and these can likewise be selected during the sewing process one after another by actuating a manual button.
However, a human operator must select in all cases a gathering value that appears suitable to obtain the desired length of the fabric layer or a range of extra width. Even though an experienced operator can sometimes do this first straightaway, she must also sew a sample seam in order to determine on the basis of the finished result whether the selected gathering value was correct or incorrect. However, an inexperienced operator will usually find the correct gathering value only after some trial and error, as a result of which time is lost and reject materials are produced.